I used to be a big baseball card collector.. while I don't have anything super valuable I do have a few nice Maris PSA 8s and Killebrew PSA 8's along with some other pre-70 named cards at lesser ratings. IMO, two things killed the baseball card collection "hobby".
1) Internet: Before ebay and such, the only place to find that 89 Griffey UD card you wanted was down at the local card store or maybe the card show in town once in a while. Most people didnt actually realize how many cards were out there - which is what someone said earlier, post 80's cards were mass produced. Once the internet hit, people had connections all over the world and realized there were tons of these cards being sold and the price dropped drastically.
2) Card grading such as PSA and BGS. Before the intorduction of card grading companies, your mint could have been my near mint and was up for debate. Now PSA and BGS basically own the definition of Mint/NearMint/Ececllent/Ect. To get a card graded requires a membership fee and a price per card fee. Most people, espicially kids, don't have the money to send to a company to grade their cards, and buying high graded cards demand a high price. The other thing, and I may be wrong about this, but I believe the card grading companies have a connection with their best clients to inflate high graded cards and keep the number of high graded cards low. For example, PSA knows just how many of my 1960 Harmon Killebrew PSA 8's are out there. Its my opinion they only award the higher grades to their best clients. When you look at my Killebrew, the thing looks freaking perfect (and I am tough on cards) however, it came back as an 8. I can always pay more to get it re-graded but who knows if it will get a 9 next time. Yes, it would increase the value by 400 or 500 bucks, but I've heard stories of people sending their cards in multiple times before they finally got the 9 or 10 they were looking for and sometimes never getting the grade they thought should have been awarded, but granted in some cases it probably shouldnt have been awarded.
Sorry, long winded response, but I used to love collecting cards when I was a kid, sad to see it disappear.